In a process for preparing chemical cellulose according to the sulfite process the debarked wood in form of chips is cooked with solutions of bisulfites (hydrogen sulfites) or sulfites, the so called cooking acid, also referred to as cooking liquor or cooking lye. Depending on whether the solutions of bisulfite contain additional sulfur dioxide or not the processes are designated as acid bisulfite processes (using excess of SO.sub.2) or bisulfite processes (without excess SO.sub.2).
As the basic component for the bisulfites, sodium or ammonium bases can be used in all processes. The use of calcium is restricted to the acid bisulfite process, and the use of magnesium is restricted to the acid bisulfite process and the bisulfite process because calcium and magnesium sulfite, respectively, precipitate at higher pH values.
The sulfite digestion is mainly carried out batchwise conducting the cooking in large, e.g. 225 m.sup.3 digesters equipped with a circulating pump for circulation of the cooking acid. Heating is effected by heat exchangers in the circulating line or by directly introducing steam.
In the one step bisulfite process after having fed chips and (cold) cooking acid into the digester the chips are impregnated with the cooking acid. After increasing the temperature to about 110.degree. C. the digester is maintained at said temperature from 2 to 4 hours, and then the temperature is increased to from 125.degree. to 150.degree. C.
Two or more step processes differ from the aforesaid process in that the real cooking step, i.e. the digestion which only takes place at a higher temperature (110.degree. C. or above) is carried out in several steps having different pH values. Examples for such processes are a process in which the digestion is first conducted under acid and then under weak acid to basic conditions and a process in which the acidity is increased after a basic to neutral step.
Furthermore, a process is known in which the digestion is started with a neutral or weakly acid step (4% SO.sub.2 ; 140.degree. C.) essentially effecting the sulfonation, followed by an acid step (5% SO.sub.2 ; 132.degree. C.).
However, the known processes do not entirely fulfill practical requirements considering, in particular the length of time and consumption of chemicals required for at the digestion and the properties of the pulp and chemical cellulose obtained.